Typically, in order to achieve high brightness and light uniformity, a backlight unit for a liquid crystal display includes a reflective film, a diffusion film, a prism film, a brightness enhancing film, etc.
In particular, a brightness enhancing film is effective at increasing light efficiency and brightness of a liquid crystal display, and specifically, a brightness enhancing film is an optical film which enables only light that vibrates in any one direction among natural light rays incident while vibrating in a variety of directions to pass threrethrough and light that vibrates in the other directions to be reflected.
Generally, a brightness enhancing film enables 50% of incident light to pass therethrough and the remaining 50% to be reflected. A conventional brightness enhancing film is configured such that the thicknesses of thin film layers in predetermined regions of a multilayered thin film for reflecting wavelengths similar to those of incident light are designed to be the same (FIG. 1), but such a brightness enhancing film is problematic because reflective light cannot be thoroughly reflected.